Ophthalmic lenses incorporating light-polarizing films are well known and have been produced for some time. Such lenses are used in sunglasses, sports goggles, fashion eyewear, and the like. The lenses can be prescription lenses with a net optical power or plano lenses with no net power, including decentered plano lenses (see, for example, Houston et al., U.S. Pat. No. 5,648,832). The light polarizing film performs the important function of reducing sun glare, including sun glare from water and snow.
Unfortunately, the incorporation of light polarizing properties into ophthalmic lenses in a cost effective and efficient manner has proved to be a technical challenge, especially for plastic lenses composed of such materials as polycarbonates, allyl diglycol carbonates (ADC), and acrylics. Various proposals directed to this problem can be found in, for example, Hovey, U.S. Pat. No. 4,043,637, Kawaki et al., U.S. Pat. No. 5,051,309, Murata et al., U.S. Pat. No. 5,702,813, Bhalakia et al., U.S. Pat. No. 5,757,459, PCT Patent Publication No. WO 97/38345, and co-pending and commonly assigned U.S. patent application Ser. No. 08/637,360, filed Apr. 25, 1996, and entitled "Polarized Plastic Laminates," the contents of all of which are incorporated herein by reference.
One method which has been used to manufacture polarized lenses involves laminating or gluing a pre-formed piece of polarized sheet to the outer convex surface of a glass or plastic lens blank which is then edged and inserted into suitable eyeglass frames. Polarized sheets for these applications have had a three layer sandwich construction, the two outer layers being cellulose acetate butyrate and the center layer being a thin polyvinyl alcohol polarizing film. The overall sheet thickness has been in the range of 0.010"-0.030". As a result, lenses produced by this approach have generally been too thick and have not been widely used. Also, their optical properties have generally been poor as a result of the thickness of the polarized sheet.
PCT Patent Publication No. WO 97/38345, referred to above, discloses a process for producing polarized lenses by laminating polarizing sheets to or between flat sheets of plastic materials such as acrylic, styrene or polycarbonate. The laminates are then heat and pressure formed to the desired lens curvature.
Light-polarizing lenses have also been produced by placing a pre-formed piece of a thin polarizing film into a glass or metal mold which is then filled with a thermosetting plastic monomer, such as an allyl diglycol carbonate (e.g., CR-39), followed by polymerization or curing to produce a lens. In this process, the monomer surrounds and thus encapsulates the polarizing film. After removal from the mold, the lens is ground, edged and finished into a prescription or sunglass lens.
Polycarbonate lenses for all types of eyewear have recently become increasingly popular because of their ease of manufacture, light weight, and high impact and shatter resistance. The increased usage of polycarbonate lenses for eyewear has been driven both by fashion and safety concerns, especially in connection with lenses which are to be worn during sports activities.
A process has been developed for the manufacture of injection molded polarized polycarbonate lenses in which a disc of pre-formed polycarbonate polarized sheet, approximately 0.028" thick, is placed into and in contact with the concave cavity of a polished steel mold which is then closed and injected with polycarbonate resin to form a lens. The molten polycarbonate fuses to the rear concave polycarbonate surface of the polarized sheet thus forming a lens.
Many problems exist with this process which result in high costs and low yields. For example, forming the polycarbonate polarized sheet is difficult and requires expensive tooling to obtain the correct curve to match the polished steel cavity. Moreover, the polycarbonate polarized sheet must be able to withstand the high temperatures necessary to injection mold the polycarbonate resin. Most polarized sheets or films decompose at these temperatures, which results in reduced polarizer efficiency and/or low manufacturing yields.
In addition, it is very difficult to mold polycarbonate lenses of this type without creating residual birefringence which reduces or destroys polarizing efficiency. In particular, excessive shrinkage of the molded "sandwich" produces birefringence in the finished lens, as well as optical distortion. In addition to the foregoing problems, the mold cavities needed for this process are expensive to build and require a high degree of maintenance.